Blowable fuses have been used in a variety of applications to control the operative features of an integrated circuit. For example, such fuses have been used for the correction of bandgap references and data converters. Usually, this is done at the wafer probe stage and requires the use of substantial off-chip equipment.
In practice, such techniques have been found to be subject to certain limitations. One such limitation is that it is often desirable to operate upon the fuses not only at the wafer probe stage, but also later, following packaging. Other limitations result from the amount of power which is required to operate (blow) such fuses. For example, the switching clement provided on the integrated circuit for blowing the fuse must in practice occupy on the order of one-half of the total area of the blowable fuse circuit. Primarily, this is because the size of the switching element which is used to blow the fuse is responsive to the amount of current required to open the fuse, which continues to present a significant limitation. In addition, the amount of power required to blow the fuse has in practice been found to be potentially damaging to the passivation layer of the integrated circuit, which can permit the passage of moisture and the corrosion which can result from such moisture.
Efforts have been made to overcome these limitations. For example, De Wit et al., "A Low-Power 12-b Analog-to-Digital Converter with On-Chip Precision Trimming", IEEE Journal of Solid-State Circuits, Vol. 28, No. 4 (April, 1993), discloses the use of blowable fuses to reduce the capacitor ratio error in a data converter circuit. Circuits for accomplishing this are further described in U.S. Pat. Nos. 5,353,028 and 5,235,335. In an effort to allow programmable, on-chip adjustments to be performed after packaging, and at a lower voltage (power) level, De Wit et al. suggest the use of poly-silicon fuses.
In addition to the need to improve upon the blowable fuses, there is a corresponding need to improve upon the operations which are used to program such fuses, while ensuring that the fuses are both safely and reliably blown. For example, De Wit et al. suggest the programming of their disclosed poly-silicon fuses using a corresponding series of switching elements (e.g., transistor switching elements). The switching elements are coupled with the fuses in pairs, and are operated responsive to data loaded into a shift register.
The state of each fuse pair (i.e., "opened" or "closed") is serially established (one at a time) responsive to operation of the associated switching elements, and the state to be established for each fulse pair is determined by the data stored in the shift register (all "ones" except for any fuses to be opened). While this results in fuses that are logic programmable, such programming is limited in operation and a more flexible means of programming the fuses therefore remains desirable.